Semiconductor-based photodetectors capable of converting an optical signal into a detectable electrical signal are widely used in diverse technical fields such as optical communication networks. Photodiodes based on a p-i-n junction, also called PIN photodiodes, are particularly suited as high speed photodetectors due to their rapid response to incident light in comparison to p-n junctions. Nonetheless, the bandwidth of conventional PIN photodetectors, i.e. the speed of response to incident light, is often limited by the much slower transit time within the intrinsic i-layer of hole carriers of an electron-holes pair in comparison to electron carriers.
A common way to increase bandwidth of the photodetector is to reduce the vertical length (i.e. height or thickness) of a light absorption layer so as to reduce the transit time of charge carriers. Since this leads directly to a larger specific capacitance, the area of the photodiode and by that the photodetecting area has to be reduced in order to keep the capacitance below a certain specified value. Consequently, at the same time, the responsivity of the photodiode decreases with a decrease in the thickness of the light absorption region. An approach for reducing the specific capacitance lies in introducing an intrinsic drift layer that does not absorb light in the wavelength range of interest. In a first approximation, the carriers generated in the light absorption layer travel to collecting layers and electrodes at drift velocities which are proportional to the electrical field and the respective carrier mobility. Generally, the carrier velocity increases with field until it saturates. However, the saturation velocity for electron carriers is typically attained at lower electric fields than for hole carriers. Hence, as in most of conventional systems the applied external electric field is limited, the hole carriers often do not reach their saturation velocity. One should notice that as holes travel slower than electrons, the illumination of photodiodes is often provided from the p-side, which results in a shorter effective distance for the holes to travel along. By designing the photodetector such that only the faster carriers (electrons) will have to travel over the additional drift layer, the drift layer will have only a minor contribution to overall transport times but will have a substantial effect on reducing the specific capacitance of the photodiode.
An approach for improving frequency response and saturation output of a PIN photodiode has been proposed in U.S. Pat. No. 5,818,096. This approach lies in separating the function of light absorption and carrier traveling between two semiconductor layers instead of using the same depleted, intrinsic semiconductor layer as in the conventional PIN photodiode. In particular, a p-type semiconductor layer is used as the light absorption layer and an intrinsic, non-absorbing semiconductor layer is used as carrier transport layer. In this configuration, the response time of the carrier injection into the carrier traveling layer is essentially determined by the diffusion time of the electrons in the p-type light absorption layer. Since the response time of hole carriers in the light absorption layer is extremely short, as they only respond to in relation to movement of the electrons within this layer, the slower drift velocity of the hole carriers in the carrier transport layer does not directly contribute to the photodiode response. This results in improved frequency response and saturation output. However, the increase in saturation power implies a lower responsivity of the photodiode.
Published patent application US 2007/0096240 A1 proposes a photodiode structure for enhancing responsivity at the cost of lower-saturation power. The proposed photodiode structure includes, in addition to the common intrinsic, light absorption layer, a p-doped layer and/or an n-doped layer as additional light absorbing layers. In this case, the movement of minority carriers (i.e. the carriers of polarity opposite to doping carriers) within the doped (undepleted) absorption layers is essentially determined by their respective diffusion time. The minority carriers may then rapidly diffuse from the doped absorption layers into the intrinsic layer and, therefore, do not significantly affect the total transit time in comparison with a conventional PIN photodiode. As the additional doped absorption layers increase the overall optical absorption volume, the photodiode responsivity is also increased.
Patent application WO 03/065416 describes a modified PIN photodiode for increasing responsivity of the device without substantially reducing bandwidth. The proposed photodiode has a p-type semiconductor layer and an n-type semiconductor layer coupled by a second p-type semiconductor layer, which acts as the light absorption layer. The second p-type semiconductor layer has a graded p-doping concentration along the path of the carriers, which varies from a high value near the anode to lower values towards the cathode. The graded p-doping concentration increases the net absorption of the photodiode without substantially reducing the transit time of the carriers within the absorption layer. Such graded doping increases capacitance relative to an intrinsic semiconductor of the same thickness, although the pseudo electric field that is created by the graded doping may give the electrons a higher velocity that compensates for the increased capacitance.